This invention relates to a papermaking machine.
As is known, papermaking machines are some of the most massive and, at the same time, most delicately balanced pieces of mechanical equipment found in any industry. In order to operate such machines over extended time periods with a minimum of downtime due to faults, the highest engineering skill in manufacture, steadiness and perfect adjustment of the speed of several separate but interlocked parts is frequently required. Generally, the pulp supplied to a papermaking machine is processed before flowing to a head box. This head box usually has a feed channel from which the pulp is delivered onto a travelling wire screen or other water pervious element. The wire screen or other water pervious element then conveys the pulp past various vacuum boxes or the like so that the liquid, such as water, may be drained from the pulp to allow the fibers within the pulp to felt together into a paper.
In one known papermaking machine use is made of a head box having a feed channel which extends along a sheet-forming zone of a moving water-pervious element, the pulp being supplied to the feed channel in a constant amount per unit of time. In addition, at least one vacuum box is provided near the sheet-forming zone on the side of the pervious element remote from the feed channel with the vacuum box being connected to a source of negative pressure. Also, a means is provided for controlling the negative pressure in the vacuum box. In such a machine, an infeed of pulp in a constant quantity per unit of time usually leads to the formation of a web of uniform thickness and density. However, the perviousness of the web which forms on the element, e.g. a cylindrically formed wire, in the sheet-forming zone may vary in dependence upon variations in the fineness (degree of grinding) of the infeed pulp. Thus the water forming the pulp suspension either issues prematurely from the feed channel through the element or, if the web is insufficiently pervious, overflows from the end of the feed channel.
Accordingly, it is an object of the invention to provide a papermaking machine in which pulp is dewatered uniformly in a sheet-forming zone of a movable water-pervious element.
It is another object of the invention to be able to detect variations in the degree of grinding of pulp delivered to a movable water-pervious element of a papermaking machine and to adjust the machine to such variations in a simple manner.
It is another object of the invention to maintain a web of uniform thickness and density on a moving water-pervious element of a papermaking machine in a simple efficient manner.
Briefly, the invention provides a papermaking machine having a head box including a feed channel for delivering pulp with a collecting sump adjacent the feed channel for receiving pulp which overflows from an end of the feed channel and with a means for determining the rate of pulp overflow into the sump in order to emit a signal representative of the rate of overflow.
In one embodiment, where the machine has a movable water-pervious element for receiving the pulp delivered from the feed channel, at least one vacuum box on a side of the water-pervious element remote from the feed channel for drawing water from the pulp on the element and a means for regulating the negative pressure in the vacuum box, this latter means is connected to the means for determining the rate of pulp overflow into the sump to receive the signal and to vary the negative pressure in the vacuum box in response to the signal so as to obtain a constant overflow rate into the sump.
In addition, where the machine has a grinding means for grinding the pulp prior to delivery to the feed channel, the grinding means may also receive the signal and vary the degree of grinding in dependence on the signal.
The means for determining the rate of pulp overflow into the sump may include a pulp return line for returning the excess pulp to a pulp feeder, or other means for supplying pulp to the feed channel, in a constant quantity per unit of time, a throttle means in the return line for controlling the flow of pulp in the line and a sensor. By properly setting the throttle means for a given rate of overflow, the level of pulp in the sump can be maintained constant. The sensor is used to detect the height of the level of pulp and to emit an observed value signal in response to the detected height.
During use, pulp is supplied to the feed channel of the head box in an excess of e.g. from 2-10% so that there is a continuous overflow into the sump. The throttle means and the sensor are then used to determine the amount of overflow per unit of time into the sump, the resulting observed value being used as a measure of web permeability. This measurement is very accurate since variations in perviousness react very precisely on the small amount of overflow. Another advantage is that the overflow keeps the end of the feed channel clean and free from fibrous accumulations.
The observed-value signal of the sensor can also serve as a control signal for control facilities associated with other parts of the papermaking machine which act on or are acted on by the degree (i.e. fineness) of grinding of the pulp. For instance, the observed-value signal can be used to adjust a refiner preparing the pulp for the feed channel, since the fineness of grinding provided by the refiner directly affects the perviousness of the web formed in the sheet-forming zone. A similar kind of control can be provided for driers and similar sections of the papermaking machine whose effect also depends upon the fineness of grinding and the resulting perviousness of the web.